Infusion units used in machines for making hot beverages, in particular coffee, normally comprise an infusion chamber delimited by at least two portions movable relative to one another. In automatic machines, opening and closing of the infusion chamber takes place by means of an actuator, for example an electrical motor which, through a suitable linkage, opens and closes the infusion chamber moving the two portions of the infusion chamber relative to one another. An example of an infusion unit of this type is described in U.S. Pat. No. 4,681,028.
For making espresso coffee, water at a high pressure, typically comprised between 10 and 18 bar, must be fed into the infusion chamber. This high pressure generates high reaction forces on the structure of the infusion unit. This structure is made so that the water pressure does not cause an accidental opening of the infusion chamber. This normally requires that the two portions defining the infusion chamber reach a well defined reciprocal closing position of the chamber. As a consequence, the infusion chamber always takes on the same position and the same volume defined by the travel end position of the members controlling the opening and closing movement.
These infusion units are especially reliable and allow achieving high infusion pressures. However, due to the impossibility of varying the volume of the infusion chamber or in any case of varying it in a very limited manner, they do not allow receiving variable amounts of coffee in the infusion chamber, or they allow a very moderate variation of this amount. Typically, about 8 grams of coffee powder are used for making a portion of espresso coffee. In the most sophisticated automatic machines, the user has the possibility of slightly changing this amount of coffee, to better satisfy his/her taste, for example for having stronger or less strong coffee. However, the range of variation is moderate. Moreover, it is not possible to dispense two coffee cups in a single infusion cycle. It is only possible to select the automatic execution of two infusion cycles in a sequence.
An infusion unit with a variable volume infusion chamber for alternately receiving one or two product pods is described in EP-A-1641371.